1. Field of the Invention
The present invention relates to the synthesis of a new class of ladder and partial ladder polymers. For reviews on ladder and partial ladder polymers, see: C. G. Overberger, J. A. Moore Adv. Polymer Sci., 1970, 7, 113; C. Arnold J. Polymer Sci.: Macromolecular Rev., 1979, 14, 265; D. K. Saltbayev, B. A. Zhabanov, L. V. Pvovarova, Polymer Sci. U.S.S.R., 1980, 21, 799. A ladder polymer comprises an uninterrupted series of rings such that a double strand is formed. (See below.) ##STR1## Ladder polymers are, intuitively, more thermally stable than the related single strand polymers since cleavage of two bonds in the same connecting ring, which is a characteristic of ladder polymers, is necessary to cause chain scission. If the bond breakage is completely random, that is, if all the connecting links are equally strong, then chain scission is less likely to occur. In addition, since the two broken segments are held close to each other after one bond ruptures, a recombination or "bond healing" of the ruptured bond is possible. An illustration of the above is as follows: ##STR2## The ladder polymer 3, a polyiptycene, (The name "iptycene" emphasizes the relationship between these compounds and the parent structure, triptycene. A prefix indicates the number of separated arene planes; thus 1, is a triptycene (three planes) and 3 is a polyiptycene (many planes). (See also Hart, H.; Shamouilian, S.; Takehira, Y.: J. Org. Chem., 1981, 46, 4427.) Polyiptycene behaves as a classical ladder since all the bridging bonds are equally strong. Additionally, since the connecting links are unstrained [2.2.2]-octabicyclic systems, bond healing is possible. Thus, polyiptycene is highly thermally stable.
The synthetic route to 3, shown above, involves a Diels-Alder addition of the photochemically generated aryne intermediate 2 to the anthracene moiety on another molecule of 1. Monomer 1 is prepared as follows. Bis-epoxide 4 reacts as a dienophile with anthracene 5 to form the one to one adduct 6. Deoxygenation to triptycene followed by side chain manipulation yields 1. ##STR3##
2. Discussion of Related Art
It has been established that 1,4,5,8-tetrahydro-1,4;5,8-diepoxyanthracene, 1, is an effective bis-dienophile for the rapid construction of fused-ring systems, (H. Hart, A. Bashir-Hashemi, J. Luo, and M. A. Meador, Tetrahedron, 42, 1641, (1986), Hart, H.; Raju, N.; Meador, M. A.; Ward, D. L.: J. Org. Chem., 1983, 48, 4357). For example, the dehydrated pentiptycene, 2, can be prepared in one step from anthracene and 1. Extension of this chemistry ##STR4## to the synthesis of ladder or partial ladder polymers can be accomplished by reacting bis-dienophile 1 with a bis-diene (W. J. Bailey, In Step-Growth Polymerizations, D. H. Solomons, Ed., Marcel Dekker, New York, 1972, Chapter 6).
Compounds such as 2 have extremely high melting points and decomposition temperatures. (M. A. Meador, Model Studies of Diels-Alder Polymers Using 1,4.5.8-Tetrahydro-l,4;5,8-Diepoxyanthracene as a Bis-dienophile, 18th Central Regional Meeting, Am. Chem. Soc., Bowling Green, OH, June 1986). Thus, polymers made from 1 and thermally stable bis-dienes have potential for high temperature applications. Although many Diels-Alder polymers have been investigated in the past, only three studies have employed anthracenes as the reactive diene (J. S. Meek, P. A. Argabright, R. D. Stacy, 134th National Meeting, Am. Chem. Soc. Chicago, IL, September 1958, abstracts, 23, M. P. Stevens, J. Polym. Sci. Polym. Lett. Ed., 22, 467 (1984)). One early study involved copolymerization of a 9,9'-linked bis-anthracene, 3, with a bis-dienophile, leading only to very low molecular weight polymers (S. Dumitrescu, M. Grigoras, and A. Natansohn, J. Polym. Sci. Polym. Lett. Ed., 17, 553 (1979)). This has been attributed to steric congestion at the site of cycloaddition, (Bailey, cited above).
A more recent report, (Stevens, cited above), described the Diels-Alder polymerization of N-(2-anthryl)maleimide, containing a reactive dienophile unit linked to the diene at a position remote from the site of cycloaddition, thus reducing steric hindrance. However, once more, only low molecular weight polymer was obtained The author speculated that this was dueto the ease of reversibility of the Diels-Alder reaction. While this may be the case, a more likely explanation is that Diels-Alder cycloaddition is competing with vinyl polymerization of the maleimide units. Any attempt to prevent this vinyl polymerization, such as by lowering the reaction temperature or using a free radical quencher, may also inhibit addition reaction. Although usually thought of as a concerted reaction, Diels-Alder cycloadditions often proceed via a two-step mechanism with either a zwitterionic or biradical intermediate, (J. Sauer and R. Sustmann, Angew, Chem. Int. Ed. Engl., 19 779, (1980)).
In this invention, the synthesis of thermally stable co-polymers from the reaction of anthracene end-capped polyimide oligomers acting as bis-dienes and bis-dienophile 1 will be discussed. The anthracenes are linked through the 2-position, remote from the site of cycloaddition, and the bis-epoxide 1 does not undergo vinyl polymerization at the same temperature that cycloaddition takes place.